For slower songs I usually completely write the song with just vocals and piano or guitar. I then add a bass line, I actually pretty much use classical harmony for that (bass on the key note unless I want an inversion, plus passing notes). I'll then jam around the bass line for a while, and if I find any nice fills or anything unusual that works, I swap it in.
I then see if I can come up with a lead guitar part. I usually try to write lead parts by just imagining sounds without playing, as that can allow you to come up with unusual ideas and rhythms. Often I'll come up with a line that I would never have written with an instrument, as I can't actually play it. If I come up with anything that fits, I layer it in.
I then get the band together and try out various drum lines. Then, most importantly, I listen to the overall sound and see if it's too busy, if anything clashes, if the sound is even, if it has the right kind of 'energy flow' etc. I then take things out and put thing in until it sounds right. Just last week I completely arranged a song, and then decided it was better with just piano.
For a more rhythmic song, I start with a basic rhythm and then just throw ideas in, arrange a couple of sections, and then sort out the structure at the end. With the drums, I often avoid having high hat on every beat, instead having the hat play another counter rhythm. I think the drums should sound almost melodic, a line that you would want to listen to by itself. I tend to use more riff based bass lines for faster songs, and often I like to match up the lead guitar to the bass, leaving second guitar or keys to fill in the chords.